Here are some important terms when it comes to working ESL students.

BICS: Basic Interpersonal Communication Skills: Language needed in social situations.
http://www.everythingesl.net/inservices/bics_calp.php

Borrowing: The incorporating of a loan word from one language into another. Ex: English borrowed buoy from dutch.

CALP:refers to formal academic learning: this includes listening, speaking, reading, and writing about subject area content material.
http://www.everythingesl.net/inservices/bics_calp.php

Codeswitching: the movement back and forth between two languages of dailects within the same sentence or discourse.
http://www.wordiq.com/definition/Code-switching

Cognates: Words in related languages that developed from the same ancestral root. Ex. English man and German mann.
http://cognates.org/

Dialect: A variety of a language whose grammer differs in systematic ways from other varieties. Differences may be lexical, phonological, syntactic, and semantic.
http://dictionary.reference.com/browse/dialect

Fossilizaion: A characteristic of second-language learning in which the learners reaches a plateau and seems unable to acquire some property of the L2 grammer.
http://en.citizendium.org/wiki/Fossilization_%28language_acquisition%29

Grammer Translation: A method of second-language learning in which the student memorizes words and syntactic rules and translates them between the native language and target language.
http://dictionary.reference.com/browse/grammar-translation+method

Idiolect: An individual's way of speaking, reflecting the person's grammer.
http://www.answers.com/topic/idiolect

Language Isolate: A natural language with no demonstrable genealogical relationship with other living languages.
http://www.wordiq.com/definition/Language_isolate

Linguistic Competence: The knowledge of a language represented by the mental grammer that accounts for speakers' linguistic ability and creativity. (mostly this in unconcious knowledge)
http://www.answers.com/topic/linguistic-competence

Loan Word: Word in one language whose origins are in another language. Ex: in Japanese, besiboru, "baseball," is a loan word from English.

Linguistic Performance: The use of linguistic competence in the production and comprehension of language. Ex: Linguistic competence permits one-million-word sentences, but linguistic performance prevents this from happening.

Polyglots: A person who speaks several languages.
http://www.thefreedictionary.com/polyglot

Simultaneous bilingualism: Refers to the simultaneous acquisition of two languages beginning in infancy (of before the age of three years).
http://dictionary.sensagent.com/simultaneous+bilingualism/en-en/

Slang: Words and phrases used in casual speech, often invented and spread by close-knit social or age groups, and fast-changing.
http://www.slangsite.com/ offers examples from a to z.